1. Technical Field
The present invention relates to a polarization conversion element that is used for converting randomly-polarized light beams emitted from a light source into one type of a polarized light beam.
2. Related Art
Projection-type video systems (projection apparatuses) such as liquid crystal projectors are devices that modulate light emitted from a light source device in accordance with image information and project a modulated optical image onto a screen in an enlarged scale.
In such projection apparatuses, in order to improve the use efficiency of light, polarization conversion elements are used for splitting light (hereinafter, referred to as random light) including randomly-polarized light beams (polarized light beams such as a P-polarized light beam and an S-polarized light beam of which the polarization planes are perpendicular to each other, light beams in which linearly polarized light beams of which the directions of polarization planes are various are mixed, circularly-polarized light beams, and elliptically-polarized light beams) emitted from a light source device into a plurality of intermediate light beams, converting the split intermediate light beams into one type of linearly-polarized light, and uniformly outputting the linearly-polarized light.
Generally, such polarization conversion elements have structures disclosed in JP-A-2000-298212 and Japanese Patent No. 3309846.
Such a polarization conversion element, generally, has a configuration in which a ½ wave plate formed from an organic material such as a polycarbonate film is bonded to an outgoing-side surface of a polarizing beam splitter (PBS) array (prism array) using an organic adhesive, which is acquired by generating a laminated body, in which a PBS film (an optical functional film a so-called polarizing splitting film having a function of transmitting one linearly-polarized light beam of the P-polarized light beam and the S-polarized light beam having an orthogonal relation and reflecting the other linearly-polarized light beam) and a reflective mirror film are formed on both principal surfaces, acquired by alternately stacking multiple light transmitting substrates such as colorless transparent glasses and cutting the laminated body at a predetermined angle, for example, 45 (degrees) (or 135 (degrees) with respect to the incident face (stacking face). Thus, random light emitted from the light source is selectively incident to the PBS film due to a light shielding plate arranged on the optical path, and is split into an S-polarized light beam and a P-polarized light beam, and for example, the P-polarized light beam is transmitted through the PBS film, and the S-polarized light beam is reflected by the PBS film.
When the P-polarized light beam transmitted through the PBS film is incident to the ½ wave plate, the phase of the P-polarized light beam is shifted by 180 (degrees) so as to be converted into an S-polarized light beam and incident from the ½ wave plate. In addition, the S-polarized light beam reflected by the PBS film is further reflected by the reflective mirror film so as to be output from an outgoing face located in an area of the PBS array in which the ½ wave plate is not arranged.
As a result, the light output from the polarization conversion element is uniformized as S-polarized light.
In contrast to the above-described case, it may be configured such that the PBS film transmits the S-polarized light beam and reflects the P-polarized light beam, the S-polarized light beam transmitted through the PBS film is converted into the P-polarized light beam through the ½ wave plate, and the P-polarized light beam reflected by the PBS film is reflected by the reflective mirror film so as to be output from the outgoing face of the PBS array in which the ½ wave plate is not arranged, whereby the light output from the polarization conversion element is uniformized as the P-polarized light.
FIG. 29 is a diagram illustrating the configuration of a general polarization conversion element.
A light transmitting substrate 98 in which a polarization splitting film 91 and a reflective (mirror) film 92 are formed and a light transmitting substrate 98 in which such films are not formed are alternately bonded together through an adhesive layer 93, the bonded laminated body is cut at a predetermined angle, for example, 45 degrees (or 135 degrees), the cut face is ground, and a phase difference plate 97 is bonded to an element main body 95, in which a light incident surface 951 and a light outgoing surface 952 are formed, through an adhesive layer 96.
Recently, in a white light source lamp as a light source used in a liquid crystal projector that uses the polarization conversion element having the above-described configuration, implementation of a high output and a short arc length has progressed, and the thermal load of the PBS array and the ½ wave plate described above has been increased.
Thus, in Japanese Patent No. 4329852, a polarization converting unit has been proposed in which a plurality of phase difference plates that are formed by using quartz crystal members and convert the polarization axis of one of linearly-polarized light beams separated by a polarization separating film into the polarization axis of the other is arranged on a light beam output side of the polarization separating element array in the polarization separating element array through a spacer member having a predetermined thickness. In addition, it is described that the spacer is configured as a double-sided tape such that the spacer member can be directly bonded to an attachment object.
However, there are problems in that an adhesive (the adhesive layer 93) that is generally used for stacking and bonding a light transmitting substrate 98 when the polarization conversion element as shown in FIG. 29 is configured does not respond to high-luminance lamp light and is degraded so as to decrease the transmittance of light.
As the reasons for the problems, the following and the like may be considered. An adhesive that is generally used is composed of a component having high absorptivity for short-wavelength light. The viscosity of the adhesive is high, and thus the coating amount thereof is large so as to thicken the adhesive layer 93, whereby the amount of absorbed light is large. In addition, the adhesive is composed of a component that has a low decomposition temperature.
In addition, in the general polarization conversion element shown in FIG. 29, the adhesive layer 93 is thickened as described above, and, in a case where the laminated body is cut in the state in which the adhesive layer 93 is thickened, distortions occur in the end portions of the adhesive layer 93. When the cut face is ground in the state in which the distortions occur, as shown in FIG. 30, the corners 981 of the light transmitting substrate 98 near the adhesive layer 93 are removed. Accordingly, a gap is generated in the adhesive layer 96 used for bonding the phase difference plate 97 to the element main body 95, and there are problems in that the phase difference plate 97 may be easily peeled off, and air bubbles 961 may be formed so as to decrease the light transmittance.
Furthermore, by removing the corners 981 of the light transmitting substrate 98 near the adhesive layer 93, there is an additional problem in that an area through which light is effectively transmitted is decreased.
Regarding such problems, in JP-A-2002-139703, in an example of a wave plate that is configured by a first wave plate and a second wave plate, a configuration is disclosed in which the first and second wave plates are not stacked but arranged to be spatially separated from each other. By applying such a configuration to the polarization conversion element, light transmitting substrates configuring the polarization conversion element can be arranged so as to be separated from each other without using an adhesive, whereby the effect of degradation of the optical characteristics of the polarization conversion element due to degradation of the adhesive or removal of the corners of the light transmitting substrate at the time of grinding can be avoided. However, in such a case, there are problems of complexity or a high cost in the manufacturing process, an increase in size of the optical element, and the like due to control of the intersection angle of optical axes of the light transmitting substrates, arrangement of an anti-reflective film formed on the front and rear faces of each wave plate, and the like, which is not desirable.
Furthermore, as a ½ wave plate (phase difference plate) used in the polarization conversion element described as above, a ½ wave plate having specifications in which a phase difference is 180 (degrees) for light of a predetermined wavelength, the polarization converting efficiency is one, and P-polarized light beam can be reliably converted into S-polarized light, or the S-polarized light beam can be reliably converted into a P polarized light beam is demanded, so that it can be applied to a liquid crystal projector that uses three wavelength bands of R, G, and B as three primary colors of light.
In addition, it is additionally required that the ½ wave plate reliably shows optical characteristics in the range of 0±10 (degrees) from the incidence angle of 0±3 (degrees) of light that is incident to the polarization conversion element.